This invention relates to improvements in optical fiber manufacture. More specifically, the invention relates to a method and apparatus for providing improved fiber drawing from an optical preform.
In the state-of-the-art optical fiber drawing from an optical preform, it is necessary to heat a portion of the preform to its "drawing temperature". A filament of glass is pulled from the heated portion of the preform to form an optical fiber using conventional heating sources such as CO.sub.2 laser, resistance or induction furnace and O.sub.2 /H.sub.2 torches. The desired light transmitting characteristics and mechanical properties can be given to the fiber by carefully regulating the diameter of the fiber drawn from the preform. In order to provide an optical fiber having uniform light transmission characteristics along its length, it is imperative that the diameter of the fiber be maintained constant along the length of the fiber. Additional factors such as the fiber drawing temperature and tension, the rate of drawing and the protection of the drawn fiber, all affect the optical characteristics of the drawn fiber.
The fiber drawing tension significantly affects the optical transmission properties of the final glass fiber. Since drawing temperature affects both optical and mechanical properties of the drawn fiber, optimum drawing temperature must be employed to obtain the ideal fiber having the desired properties. The drawing temperature is associated with other fiber drawing parameters, such as drawing speed, preform and fiber diameter, and preform feed speed. Therefore, the fiber drawing tension, which is mainly dependent on the drawing temperature, should be carefully controlled. So, for instance, drawing tension approaching 50 grams may be used for drawing very low loss optical fibers.
On the other hand, low drawing tension, for instance, of less than 5 grams may be maintained during drawing of long lengths of high strength fiber. The low drawing tension is achieved by utilizing a high drawing temperature. However, high drawing temperature causes a high degree of silica vaporization. The upper limit of the drawing temperature with regard to optimum drawing tension depends on drawing conditions.
It is important in drawing optical fibers to assure that the heat flux into the preform at the portion from which the fiber is being drawn be as constant as possible not only in the course of the drawing operation, but also from point to point of the circumference of the preform at the respective cross section, since the viscosity and thus the flow rate of the material of the preform into the fiber is dependent thereon. Should the heat flux into the preform and thus the temperature and viscosity of the material of the preform vary in the circumferential direction of the particular cross section, the cross section of the drawn fiber would be irregular rather than circular, with attendant deterioration of the light transmitting properties of the fiber. A clean drawing atmosphere should be maintained by using a clean room facility which is frequently inspected using a dust particle counter. One of the sources of contaminations on the fiber surface resulting in fiber strength degradation may be the heat source itself which may generate foreign particles from heating elements such as carbon and zirconia particles in commonly widely-used resistance or induction furnaces. A typical economical clean heat source is an oxyhydrogen flame torch in which filtered oxygen and hydrogen gases are mixed and ignited to form a oxyhydrogen flame. The torch configuration is optimized for providing uniform heat flux resulting in the formation of a neck-down portion during fiber drawing using relatively large outer diameter preforms.
In conventional constructions of drawing arrangements using the oxygydrogen flame torch, heat flux from the flame into the preform is irregular, owing both to environmental influences, such as ambient air currents, and to the shape of the flame. To ameliorate this drawback, it has been proposed to use a plurality of oxyhydrogen flame torches which are distributed around the axis of the preform and directed against the portion which is to be heated to make the material thereof flowable for the drawing operation. Yet, even here, the heat flux into the preform varies from one point of the circumference to the other; therefore relative angular displacement about the drawing axis is often conducted between the preform and the torch or torches, so that the amount of heat transmitted to the preform at the particular point of its circumference is averaged over time. However, the need for such relative angular displacement complicates the structure of the drawing arrangement and brings about other problems which are reflected in the quality of the drawn fiber.